Mechanical power is commonly recovered from external heat sources, such as combustion products, in a Rankine Cycle system, using steam as the working fluid. However, in recent years, as interest has grown in using heat sources at lower temperatures for power recovery, there has been a growing trend to look for alternative working fluids and for heat sources at temperatures of less than about 200° C. In most cases, it has been shown that organic fluids such as light hydrocarbons or common refrigerants are appropriate. These fluids have unique properties and much of the art of getting the best system for power recovery from a given heat source is based on the choice of the most suitable fluid.
Those fluids most commonly used, or considered, are either common refrigerants, such as R124 (Chlorotetrafluorethane), R134a (Tetrafluoroethane) or R245fa (1,1,1,3,3-Pentafluoropropane), or light hydrocarbons such as isoButane, n-Butane, isoPentane and n-Pentane. Some systems incorporate highly stable thermal fluids, such as the Dowtherms and Therminols, but the very high critical temperatures of these fluids create a number of problems in system design which lead to high cost solutions.
There are, however, numerous sources of heat, mainly in the form of combustion products, already used for other processes, such as the exhaust gases of internal combustion (IC) engines, where the temperatures are rather higher, typically having initial values in the range 200°-700° C., where organic working fluids are associated with thermal stability problems and their thermodynamic properties are less advantageous. Unfortunately, at these temperatures, conventional steam cycles also have serious deficiencies.
Russian patent publication no. RU2050441 discloses a method of producing electrical power by recovering energy from steam that is available as a waste product produced by an industrial process. The dryness fraction of the steam is maintained in the range of 0.6 to 1, hence the steam is relatively dry. The expansion of steam may be carried out in a twin screw machine.